It is conventional to provide a backup battery as an auxiliary battery in addition to a main battery as a power source, which supplies electric power to various devices. For example, a patent document (JP 3070842) discloses a vehicular emergency notification system terminal device, which receives electric power supply from a backup battery in place of a main battery in case that the main battery cannot supply electric power.
A vehicular backup battery, which supplies electric power of about 1,000 mAh, is required to be operable under a temperature condition of, for example, between about −30° C. to about +85° C. and have long operating life. A lithium primary battery is operable under a lower temperature condition, because it uses organic solvent, which has a lower melting point than that of water. Further, it has less self-discharge and long operating life. For this reasons, a lithium primary battery is mostly used as a backup battery for a vehicle. The lithium primary battery used as a backup battery, however, need be exchanged each time its operating life ends, because its operating life is not so long as an operating life of a vehicle.
Another patent document (JP 4346881) discloses a DC electric power apparatus for communication, which is provided for example at a base station for radio communication and includes a nickel hydride secondary battery as a backup battery for electric power failure time.
However, the following drawback arises when the lithium primary battery is used as a backup battery for in-vehicle devices mounted in a vehicle. First, a location of mounting the lithium primary battery is limited, because there are not many locations available for easy battery exchange in a vehicle in case that it is mounted in a vehicle as a backup battery. Second, a relatively large-sized battery is needed and hence its cost necessarily becomes high. Specifically, it is necessary to monitor self-discharging condition of the backup battery and periodically monitor a discharge voltage characteristic under a rated load operation, which consumes certain current consumption for detecting failure. The backup battery accordingly needs to have an initial electric capacity, which allows current consumption for monitoring operations. Thus, the backup battery need be relatively large-sized.
It is proposed to use a secondary battery as a backup battery for in-vehicle devices mounted in a vehicle for countering the foregoing drawbacks. Since a battery internal resistance increases under low temperatures, a secondary battery is used and maintained at a fully-charged condition as much as possible so that sufficient electric capacity is ensured. However, if the secondary battery, which is fully-charged, is used under high temperature conditions, the battery internal condition tends to deteriorate earlier than in a case of half-charged. For this reason, it is difficult to ensure as many durable years or operating life (for example, about 12 years) as that of a vehicle.